1. Field of the Invention
The present invention relates to a pixel circuit of an organic light emitting display.
2. Discussion of the Related Art
Recently, as multimedia applications and their use increase, the more important the flat panel displays (FPD) become. Hence, various flat panel displays such as a liquid crystal display (LCD), a plasma display panel (PDP) or an organic light emitting display are used more and more.
The organic light emitting display has rapid response time, low power consumption, and self-emission structure. Furthermore, the organic light emitting display has a wide viewing angle, so that it can excellently display a moving picture regardless of the size of the screen or the position of a viewer. Because the organic light emitting display may be manufactured in low temperature environment and by using a semiconductor fabrication process, the organic light emitting display has a simple manufacturing process. Hence, the organic light emitting display is attractive as a next generation display.
Generally, the organic light emitting display emits light by electrically exciting an organic compound. To display a predetermined image, the organic light emitting display has N×M organic light emitting diodes arranged in a matrix format and may be voltage driven or current driven. The driving methods of the organic light emitting display include a passive type and an active type using a thin film transistor.
In the passive type, an anode electrode is at right angles to a cathode electrode. The anode electrode is selected by a scan signal and the cathode electrode receives a data signal, so that the OLED emits light according to the data signal applied between the cathode electrode and the anode electrode.
In the active type, the thin film transistor is connected to an ITO (Indium Tin Oxide) electrode and a gate electrode of the thin film transistor is connected to capacitor, so that the OLED emits light according to a voltage stored in the capacitor.
FIG. 1 is block diagram showing a conventional organic light emitting display.
Referring to FIG. 1, the organic light emitting display has a display panel 110, a scan driver 120, a data driver 130, a controller 140 and power supply 150.
The display panel 110 has data lines D1-Dm, scan lines S1-Sn, and pixel circuits P11-Pnm. The data lines D1-Dm are arranged in a first direction and cross the scan lines S1-Sn arranged in second direction. The pixel circuits P11-Pnm are disposed at pixel regions defined by the data lines D1-Dm and the scan lines S1-Sn.
The controller 140 outputs a control signal to the scan driver 120, the data driver and the power supply 150.
The power supply 150 outputs voltages required to the scan driver 120, the data driver and the display panel 110 according to control signals from the controller 140.
The scan driver 120 outputs a scan signal to the scan lines S1-Sn connected to the scan driver 120 according to the control signal of the controller 140. Hence, the pixel circuits P11-Pnm of the display panel 110 are selected by the scan signal.
The data driver 130 is synchronized with the scan signal output from the scan driver 120 according to the controller 140, so that the data driver 130 applies a data signal to the pixel circuit P1-Pnm through the data lines D1-Dm connected to the data driver 130. Hence, the display panel 110 displays predetermined image by light-emitting operation of the pixel circuits P1-Pnm in response to the data signal.
FIG. 2 is circuit diagram showing a pixel circuit of a conventional organic light emitting display.
Referring to FIG. 2, the pixel circuit includes a switching transistor MS, a capacitor Cgs, a driving transistor MD and an OLED(Organic Light Emitting Diode). The switching transistor MS transmits a data signal from a data line Dm in response to a scan signal of a scan line Sn. The data signal through the switching transistor MS is stored in the capacitor Cgs. The data signal stored in the capacitor Cgs is used in generating a driving current for the driving transistor MD. Hence, the OLED performs light-emitting operation according to the driving current.
The driving current IOLED flowing through the OLED is shown by the following equation 1.
                              I          OLED                =                              1            2                    ⁢                                    K              ⁡                              (                                  Vgs                  -                  Vth                                )                                      2                                              Equation        ⁢                                  ⁢        1            
Vgs denotes source-gate voltage of the driving transistor, and Vth denotes threshold voltage of the driving transistor.
The organic light emitting display of the pixel circuit is an active matrix type and may control brightness by the driving current IOLED flowing through the OLED. Hence, uniformity of a thin film transistors, threshold voltages Vth of the thin film transistors and mobility of charge carriers should be achieved in order to have a uniform display.
The thin film transistor used in the organic light emitting display may be formed by using amorphous silicon or low temperature poly-silicon. The poly-silicon has 100 to 200 times larger electron mobility than that of the amorphous silicon, so that the thin film transistor using the poly-silicon is needed to the organic light emitting display in order to have high switching speed.
The poly-silicon may be manufactured by crystallization of the amorphous silicon, using an eximer laser to anneal the amorphous silicon. When the amorphous silicon is crystallized, grain size of the poly-silicon may not be uniform due to non-uniformity of the pulse amplitude produced by the eximer laser. Hence, each thin film transistor has different characteristics, so that each pixel may have a different brightness for the same gray scale.